One practical reason for the lack of roads is the object limit in the terrain. A road segment would be an object (has to be to have any effect on the game engine), and to place it would require many segments to go from base to base.
Another option would be to have the vehicle speed tied to the texture mapping of the spot the vehicle is on. Dark green (for forested areas) would limit tanks to a much lower speed than light green (representing let's say open prairie). Grey, which represents rocky, broken terrain would be essentially impassible to vehicles. Then you could create a special terrain skin with road segments that could be mixed and matched to "draw" the road from point A to point B. These textures would allow vehicles to reach their max road speed. I suggested a similar tie between terrain textures and difficulty of ditching airplanes.
It should be remembered that the reason we have a network of vehicle bases, with their attendant spawn points, is to simulate the road and rail systems used to bring armored vehicles to within engagement distance of the enemy quickly and efficiently. Tanks are never driven somewhere when there's a railroad around. They break down too easily, and their transmissions were the weakest link in the chain, so to speak. Likewise, they use roads where available; not just to save time, but to save ware and tear.
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Sabre, a.k.a. Rojo
(S-2, The Buccaneers)
